Heat treatment refers to the final residual force after surviving artifacts stress by heat treatment, the shape of the workpiece, & 127; the size and performance have a very important influence. When it exceeds the yield strength of the material, & 127; will cause deformation of the workpiece, the workpiece cracking will exceed the ultimate strength of the material, which is its harmful side, should be reduced and eliminated. However, under certain conditions, to make a reasonable distribution of controlled stress, it can improve the performance and service life of mechanical parts, it becomes harmful as beneficial. Analysis of the distribution and variation of steel stress during the heat treatment, so that a reasonable distribution to improve product quality has a profound practical significance. Such as the impact on the rational distribution of the surface residual stress problem for the life of parts has attracted widespread attention.

First, the heat treatment of steel stress

Workpiece in the heating and cooling process, due to inconsistencies and center surface cooling rate and time to form a temperature difference, it will lead to uneven volume expansion and contraction stress, the thermal stress. Under thermal stress due to a temperature below the surface began to Heart Ministry, the Ministry of contraction is also greater than leaving the center core portion is pulled, when the end of the cooling, since the central portion finally cooled volumetric shrinkage can not freely leaving the surface pressure on core section tension. That is the role of thermal stress and finally to the workpiece surface under pressure and heart Ministry tension. This phenomenon is cooled velocity, material composition and heat treatment and other factors. When cooled, the faster, the higher the carbon content and alloy composition, uneven cooling during plastic deformation under thermal stress generated by the larger, the final form of the residual stress is greater. On the other hand the steel during heat treatment changes due to tissue i.e. when austenite to martensite, accompanied by an increase in specific volume due to the volume expansion of the workpiece, & 127; all parts of the workpiece has a phase change, resulting in inconsistent volume grew produce organizational stress. The end result is a surface tissue stress tensile stress, the center compressive stress, and thermal stress exactly the opposite. The stress of the size of the workpiece in the martensite cooling rate zone factor, shape, chemical composition of the material.

Practice has proved that any of the workpiece during the heat treatment, & 127; as long as there is a phase change, heat stress and stress will occur. & 127; only the thermal stress in the previous organizational changes it has produced, and the stress is generated in the process of transformation organizations throughout the cooling process, heat stress and organizational stress combined result, & 127; is a piece of real the presence of stress. The results of these two combined effects of stress is very complex, being affected by many factors, such as composition, shape, heat treatment and so on. There are only two types, namely, thermal stress and stress the role of the two opposite directions offset its development process, the role of the two mutually superimposed same direction. Whether or cancel each other mutual superposition, two stress should be a dominant factor, the role of the dominant result of heat stress when the central portion of the workpiece tension, surface pressure. & 127; organizational stress results when the dominant center section Compression workpiece surface tension.

Second, the impact of stress on the hardening heat treatment crack

Present on different parts of the quenching member can cause stress concentration factors (including metallurgical defects included), produce quenching cracks have promoted, but only in the tensile stress field (& 127; especially in the maximum tensile stress) was It will be manifested, & 127; if not promote stress crack in the floor.

Quench speed is an important factor in a quenching quality can affect and determine the residual stress, it can also impart an important and even decisive influence on the quenching cracks factor. In order to achieve the purpose of quenching, usually it must be accelerated in a high temperature part cooling rate segment, make it more than the critical quenching rate of the steel to obtain martensite. On the residual stress in terms of doing counteract stress due to increase tissue thermal stress, it can reduce the tensile stress on the workpiece surface to inhibit longitudinal purposes. The effect will vary with temperature to accelerate the cooling rate increases. Moreover, in the case of energy through hardened, the larger cross-sectional dimensions of the workpiece, although the actual cooling rate more slowly, but instead of cracking the greater the risk. All this is due to the thermal stress of these steels increases with the size of the actual cooling slows down, the thermal stress is reduced, & 127; the stress increases with increasing the size of the final formation of the stress mainly tensile stress In the role of the characteristics of the workpiece surface it caused. And the slower the cooling, the smaller of the traditional concept of stress differ. For this type of steel, under normal conditions of high quench hardenability of steel can be formed in longitudinal. Avoid reliable quenching crack principle is to try to minimize the cross-section and outside the martensitic transformation of unequal. Martensitic transformation of the region to implement only mild cooling sufficient to prevent formation of longitudinal cracking. Under normal circumstances can only be produced in a non-member hardenability arc crack, although the overall rapid cooling to form the necessary conditions, but its real causes, not in the rapid cooling (including martensite district) itself, but hardened LOCAL position (determined by the geometry), the critical temperature zone temperature cooling rate was significantly slowed down, so there is no hardening due to & 127 ;. Produced in large non-hardenability member transverse and longitudinal split is determined by the residual tensile stress to heat stress as the main ingredient in hardened member center & 127 ;, but at the end of the quenching cross section of the center pieces of hardened, the first crack is formed by extension caused the inside out. In order to avoid such cracks, tend to use water - oil double quenching process. In this process, the implementation of rapid cooling temperature segment, the purpose is to ensure that only the outer layer of the metal to be martensite, & 127; and from the perspective of stress, when the fast-cooling harm than good. Secondly, the purpose of cooling the late slow cooling, not primarily in order to reduce the expansion rate and stress martensite phase transformation, but rather to minimize the cross-sectional and sectional central portion of the temperature difference between the metal shrinkage rate, thereby reducing the stress and ultimately achieve suppressing quench cracking purposes.